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1) Small drones can be detected. I'm not gonna go into the specifics, but Orlan sized drones with 3m wingspan can be detected from hundreds of km. Smaller sized commercial drones, which are 1/10th of that size, can also be detected pretty well. 2) No mater how small these drones are, they are dependent on some nav and coms system. Even autonomous "fire-and-forget" drones need a somewhat robust GPS link for navigation. For operated drones, any telemetry can and will be linked to. But, ok, let's assume some futuristic drone that has a powerful AI system to do all its navigation via onboard sensors, which do not transmit or receive any information. How could such a drone get past a radar system? By either being too small for the radar to detect, fly too low for the radar to detect, or have some geometry that voids detection. Or the radar gets jammed, while the drone tries to get past it. 3) Drones have features which can be detected by radar. Motors, for example, would be one of those. 4) Radars are rarely the only sensors used. You have a whole array of different sensors which can be used to pickup stuff. Even with the radars themselves, you could have one radar for detection / target acquisition, and another radar for precise imaging. There's no free lunch, though. A very small drone would mean limited range and payload, which in turn means you'll either have to deploy it close to the enemy, or via some larger craft. Flying a drone too close to ground ads tons of interference to the drone, not to mention detection by things like acoustic sensors, humans, cameras, and what not. But that also goes for the radars. Small targets can easily disappear in clutter, or dip under the elevation of the radar. |
GPS is a satellite "broadcast" that GPS receivers listen to. GPS receivers are "passive" in a sense that they do not transmit any signals at all.